Investigations on chemical vapour transport of intermetallic phases in the system Co/Fe/Si

Article abstract of DOI:10.1002/zaac.200300117

The ternary phases existing on the quasi binary section CoSi/FeSi and CoSi₂/β-FeSi₂ have been investigated by solid state reactions and chemical transport. The solid solution serie Co_xFe _1-xSi can be described as a regular solution. The transport behaviour calculated is in good agreement with the experiments. The phases have been characterized by X-ray powder diffraction, EDX and ICP-OES. The temperature dependence of the resistivity has been measured from 20 K up to room temperature on single crystals.

Full text of DOI:10.1002/zaac.200300117

Investigations on Chemical Vapour Transport of Intermetallic Phases in the
System Co/Fe/Si
R. Boldt, W. Reichelt*, O. Bosholm, and H. Oppermann
Dresden, Institut für Anorganische Chemie der Technischen Universität
Received August 21st, 2002.
Abstract. The ternary phases existing on the quasi binary section
CoSi/FeSi and CoSi2/β-FeSi2 have been investigated by solid state re-
actions and chemical transport. The solid solution serie CoxFe1-xSi
can be described as a regular solution. The transport behaviour cal-
culated is in good agreement with the experiments. The phases have
been characterized by X-ray powder diffraction, EDX and ICP-OES.
The temperature dependence of the resistivity has been measured
from 20 K up to room temperature on single crystals.
Keywords: Ternary silicides; Chemical transport; Mixed crystals;
Resistivity measurements
Untersuchungen zum chemischen Transport intermetallischer Phasen im
System Co/Fe/Si
Inhaltsübersicht. Im System Co/Fe/Si wurden die existierenden in-
termetallischen Verbindungen auf den quasibinären Schnitten FeSi/
CoSi und β-FeSi2/CoSi2 ermittelt und deren chemisches Transport-
verhalten untersucht. Die thermodynamische Beschreibung der
Mischkristallreihe CoxFe1-xSi als reguläre Lösung beschreibt das
experimentell beobachtete Transportverhalten hinreichend gut. Die
erhaltenen Phasen wurden mittels Röntgenbeugung, ESMA und
ICP-OES untersucht. An Einkristallen wurden elektrische Leitfä-
higkeitsmessungen von 20 K bis Raumtemperatur durchgeführt.
1 Introduction
2 The System Co/Fe/Si
Two series of mixed crystals are known in the system Co/
Fe/Si, one between CoSi and FeSi, the other between CoSi2
and β-FeSi2. The two binary compounds CoSi and FeSi
crystallize in the cubic system in space group P213 (No.198)
and form a complete solid-solution series. CoSi2 crystallizes
The preparation of intermetallic phases as a phase-pure po-
lycrystalline material and as single crystals is published only
for a few systems so far. Both the reactivity and the equilib-
rium conditions are largely unknown for the system Co/
Fe/Si. Therefore, systematical investigations are necessary
concerning the exact composition of phases and the coexist-
ence relations between them. Using the experiences on
preparation of the binary systems Fe-Si and CoSi [1, 2],
both solid state reactions and chemical vapour transport
with I2 as transport agent should by used for defined prep-
aration in the quasi-binary systems CoSiϪFeSi and
CoSi2Ϫβ-FeSi2. Looking for homogeneity ranges, mixtures
of coexisting phases should be investigated. The exact com-
position of the phases has been analysed by X-ray diffrac-
tion, EDX and ICP-OES.
¯
in the CaF2-structure type (cubic space group Fm3m,
No. 225). FeSi2 exists in two modifications: the β-phase up
to 1255 K (orthorhombic space group Cmca, No. 64), the
α-phase is stable from 1210 K to 1493 K (FeSi2.43, tetra-
gonal space group P4/mmm, No.123). Both α- and β-FeSi2
coexist between 1210 K and 1255 K. Figure 1 shows an iso-
thermal section of the system using data from the literature
[3, 4].
The mutual solubility of CoSi2 and β-FeSi2 is limited [3,
5].
A computer controlled transport balance is used for con-
tinuously measuring rates of transport in closed ampoules.
3 Synthesis and Characterization of the Ternary
Phases
Powders of Fe, Si, and Co with a purity of 99.95 % have
been used for preparation.
* Doz. Dr. W. Reichelt
Institut für Anorganische Chemie
der Technischen Universität
Mommsenstraße 13
D-01062 Dresden
FAX: 0351/463-37287
To make ternary phases exist, appropriate mixtures of
powders were placed together with small amounts of I2
(0.5-1 mg, mineralisator) in a silica ampoule. The ampoules
have been evacuated, sealed off and tempered at 1173 K for
about 10 days.
E-mail: werner.reichelt@chemie.tu-dresden.de
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R. Boldt, W. Reichelt, O. Bosholm, H. Oppermann
Fig. 3 Range of CoxFe1-xSi1 mixed crystals
y
Table 1 Phase analysis and lattice parameter in the Co-Fe-Si system
after solid state reaction with reactive gas phase (I2 )
MK1 ϭ mixed crystal CoxFe1-xSi1
y
Initial composition
Phases in equilibrium
Lattice parameter/pm
Co0.25Fe0.75Si0.85
Co0.25Fe0.75Si0.9
Co0.25Fe0.75Si1.0
Co0,25Fe0,75Si1,05
Co0.25Fe0.75Si1.1
Co0.5Fe0.5Si0.85
Co0.5Fe0.5Si0.9
MK1 ϩ Fe3Si ϩ Fe5Si3
MK1
MK1
MK1 ϩ β-FeSi2
MK1 ϩ β-FeSi2 ϩ CoSi2
MK1 ϩ Co2Si
MK1
a ϭ 447,8
a ϭ 447,9
Fig. 1 Isothermal section of the system Co-Fe-Si according to lite-
rature data [3Ϫ5]
a ϭ 446,8
a ϭ 446,9
Co0.5Fe0.5Si1.0
MK1
Co0,5Fe0,5Si1,02
Co0.5Fe0.5Si1.1
Co0.75Fe0.25Si0.9
Co0.75Fe0.25Si1,0
Co0.75Fe0.25Si1.02
Co0.75Fe0.25Si1.1
MK1 ϩ CoSi2
MK1 ϩ CoSi2
MK1 ϩ Co2Si
MK1
MK1
MK1 ϩ CoSi2
a ϭ 445,7
a ϭ 445,9
of CoSi1 and FeSi1 could be extracted from the binary
y
y
phase diagrams [4].
To investigate the real homogeneity range of
CoxFe1-xSi1 y, samples with the same Co/Fe ratio and dif-
ferent amounts of silicon have been prepared as described
before. The results of phase analysis obtained by X-ray are
shown in Table 1 and Figure 3, respectively. Single phase
samples are marked by crosses, see also Table 1 (MK1). The
results show the cobalt rich part in good agreement with
the range assumed. At the iron rich part the homogeneity
range is shifted to silicon poor regions and possibly larger
then assumed. To obtain more exact results further investi-
gations are necessary. The coexisting phases are in agree-
ment with the phase diagram, except for the observation of
MK1ϩ Fe3Si and Fe5Si3 (Fig. 1). No equilibrium could be
reached in the last case. The powder pattern of MK1 also
shows a shift with variation of the silicon content. The re-
sulting lattice constants are also given by unfilled squares
in Figure 2.
Fig. 2 Course of the lattice constants for the mixed crystals
CoxFe1-xSi
3.1 The system CoSi/FeSi
The samples prepared on the quasi binary line CoSi/FeSi
have been characterized by X-ray diffraction. The com-
pounds CoxFe1-xSi were pure phase. No traces of other
compounds could be detected. The powder patterns were
refined using the Rietveld-method and the lattice parameter
a were calculated for the cubic unit cell. Figure 2 shows the
course of the lattice constants for CoxFe1-xSi. The lattice
constants change about linear in accord with Vegard’s rule.
The expected and in literature described complete solid
solutions [5] are confirmed.
3.2 The system CoSi2/β-FeSi2
Wittmann et al. [5] have investigated the mutual solubility
of some disilicides on molten samples. They observed that
the solubility of CoSi2 and β-FeSi2 is very limited; CoSi2
dissolves 6 Mol% FeSi2, FeSi2 dissolves 1 Mol% CoSi2 [5].
We could prepare single phase ternary compounds by
solid state reactions only on the cobalt-rich side in regions
between CoSi2 until Co0.93Fe0.07Si2 (MK2). On the iron rich
side multiphase samples have always been found. The X-ray
pattern of the prepared disilicides (MK2) have been refined
by the Rietveld method and the lattice parameter could be
In the literature [4], homogeneity ranges for CoSi1 and
y
for FeSi1 y are indicated. Therefore, it is likely to assume an
analogous homogeneity range for the solid solution with
respect to the silicium content. The assumed range for
CoxFe1-xSi1 is gray colored in Figure 3. The phase ranges
y
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Chemical Vapour Transport of Intermetallic Phases in the System Co/Fe/Si
Table 2 Phase analysis and lattice parameter in the Co-Fe-Si system
after solid state reaction with reactive gas phase (I2)
MK2 ϭ mixed crystal CoxFe1-xSi2
Initial composition Phases in equilibrium
Lattice parameter/pm
CoSi2
CoSi2
a ϭ 536,20
a ϭ 536,38
a ϭ 536,55
Co0.95Fe0.05Si2
Co0.93Fe0.07Si2
Co0.75Fe0.25Si2
Co0.25Fe0.75Si2
α-FeSi2ϩMK1ϩMK2
α-FeSi2ϩβ-FeSi2ϩMK1ϩMK2
Fig. 5 Powder diffraction patterns of ground CoxFe1-xSi crystals
Table 3 Average transport rate (TR) and composition of deposited
mixed crystals
Initial
TRaver. ESMA
X-Ray, a/pm
ICP-OES
composition mg/h
Depos. Resulting
crystals. composition.
FeSi
1,4
Ϫ
FeSi0,99
Co0,1Fe0,9Si 1,6
Co0,2Fe0,8Si 1,5
Co0,3Fe0,7Si 0,6
Co0,4Fe0,6Si 0,9
Co0,5Fe0,5Si 1,3
Co0,6Fe0,4Si 0,5
Co0,7Fe0,3Si 0,4
Co0,8Fe0,2Si 0,3
Co0,9Fe0,1Si 0,2
Co0,1Fe0,9Si
448,6
Co0,09Fe0,91Si Co0,02Fe0,99Si0,99
Co0,09Fe0,91Si
Co0,18Fe0,82Si
Co0,14Fe0,86Si
Co0,07Fe0,93Si Co0,14Fe0,87Si
Co0,16Fe0,84Si
Co0,32Fe0,68Si Co0,27Fe0,73Si1,03
Co0,41Fe0,59Si
Co0,64Fe0,36Si
Co0,04Fe0,96Si 448,6
Co0,24Fe0,76Si 448,2
Co0,16Fe0,84Si 448,4
Co0,08Fe0,92Si 448,7
Co0,42Fe0,58Si 448,3
Co0,28Fe0,72Si 447,6
Co0,54Fe0,46Si 447,2
Co0,84Fe0,16Si 446,2
Ϫ
Fig. 4 Single crystal of CoxFe1-xSi obtained by chemical transport
with iodine.
calculated for the cubic unit cell (CoSi2). The results are
given in Table 2.
CoSi
0,2
CoSi0,94
The lattice constant of CoSi2 increases by substitution
with iron in the lattice. From the initial composition of
more than 7 Mol% β-FeSi2, the equilibrium state is unclear
so far. Always α-FeSi2 has been found as a second phase.
From the initial composition Co0,75Fe0,25Si2 additional
the gaseous phase. The transport takes place from lower to
higher temperature as also observed for the binary phases.
Figure 4 presents an electron micrograph of a single crystal
prepared by chemical vapour transport of Co0.4Fe0.6Si. The
chemical vapour transport has been performed from 973 K
to 1173 K with 12 mg I2 as transport agent. The aim of the
transport experiments was to obtain single crystals of the
solid solution series CoxFe1-xSi. The homogeneity range
concerning the silicon content, CoxFe1-xSi1 y, is not in our
focus.
The average transport rates for the system Co/Fe/Si are
given in Table 3. With an increasing amount of cobalt the
average transport rates decrease from 1.4 mg/h to 0,2 mg/h.
The diffraction patterns of ground CoxFe1-xSi single crys-
tals demonstrate a systematic shift of the lattice parameters.
Because of the substitution of iron by cobalt in the lattice
of FeSi, there is a shift of the reflections to higher diffrac-
tion angles as shown in Fig. 5.
CoxFe1-xSi1 (MK1) has been found, and from the initial
y
composition Co0.25Fe0.75Si2 additional β-FeSi2 has been
found (Table 2).
4 Chemical Vapour Transport and Characterization
Single crystals of ternary silicides should be obtainable by
chemical vapour transport. Using own experiences on
transport of the binary phases and results from literature
[1, 2, 6Ϫ8] iodine was chosen as the transport agent. Ap-
propriate mixtures of the elements have been used for vap-
our transport of disilicides, while single phase powders from
the solid state reaction are used for the transport of mono-
silicides. Usually, after a 24 h preheating period in a re-
versed temperature gradient, the ampoule remains for about
10 days at the transport temperature.
A comparison between the initial composition and the
deposited composition always shows an enrichment of iron
in the deposited crystals at the transport conditions de-
scribed. Therefore, a lowering of the iron content take place
in the starting material.
4.1 The system CoSi/FeSi
Investigations of chemical vapour transport have shown
that single crystals of ternary silicides can be obtained from
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R. Boldt, W. Reichelt, O. Bosholm, H. Oppermann
Table 4 Thermodynamic data of mixed crystals CoxFe1-xSi
Formula
H°298
cal/mol
∆
MH°
S°298
cal/(mol K)
Cp°
A
∆MS°
cal/(mol K)
cal/mol
B*103
C*10Ϫ5
Ω ϭ 10000
cal/(mol K)
FeSi
Ϫ18500
Ϫ18951
Ϫ19402
Ϫ19853
Ϫ20304
Ϫ20755
Ϫ21206
Ϫ21657
Ϫ22108
Ϫ22559
Ϫ23010
0
11
10,63/ 3,537/0
10,74/3,472
10,85/3,408
10,96/3,343
11,07/3,278
11,19/3,214
11,30/3,149
11,41/3,084
11,52/3,019
11,63/2,955
11,74/ 2,89/ 0
0
Fe09Co01Si
Fe08Co02Si
Fe07Co03Si
Fe06Co04Si
Fe05Co05Si
Fe04Co06Si
Fe03Co07Si
Fe02Co08Si
Fe01Co09Si
CoSi
Ϫ900
Ϫ1600
Ϫ2100
Ϫ2400
Ϫ2500
Ϫ2400
Ϫ2100
Ϫ1600
Ϫ900
0
11,576
11,856
12,008
12,063
12,035
11,927
11,736
11,448
11,032
10,32
Ϫ0,644
Ϫ0,992
Ϫ1,212
Ϫ1,335
Ϫ1,375
Ϫ1,335
Ϫ1,212
Ϫ0,992
Ϫ0,644
0
To describe the substitution of cobalt in the lattice of
FeSi exactly, the X-ray diffraction diagrams have been re-
fined by the Rietveld method and the lattice constants were
obtained from that procedure. The results are given in Table
3 and Figure 6b. The course of iron enrichment in depen-
dence on the initial composition is recognizable. EDX
analysis and ICP-OES have been used to confirm these re-
sults, and the EDX analysis confirm the results obtained by
X-ray except for some deviations which are difficult to de-
clare. The X-ray and EDX results are based on the com-
pounds CoxFe1-xSi.
From selected transport experiments five monocrystals
were dissolved in HF and analysed with ICP-OES (Table
3). The results agree well with data obtained with other
methods, but further investigations are necessary. The ICP-
OES allows a precise measurement of the silicon content.
The results confirm the homogeneity range assumed by us.
Using the equations (1)-(7), thermodynamic data of the
CoxFe1-xSi compounds could be obtained. Tabel 4 shows
the data used for calculations.
G°(CoxFe1-xSi) ϭ xG°(CoSi) ϩ (1-x)G°(FeSi) ϩ ∆MG°
H°(CoxFe1-xSi) ϭ xH°(CoSi) ϩ (1-x)H°(FeSi) ϩ ∆MH°
S°(CoxFe1-xSi) ϭ xS°(CoSi) ϩ (1-x)S°(FeSi) ϩ ∆MS°
Cp°(CoxFe1-xSi) ϭ A ϩ B10Ϫ3T ϩ C105TϪ2
A(CoxFe1-xSi) ϭ xA(CoSi) ϩ (1-x)A(FeSi)
∆MS° ϭ xRlnx ϩ (1-x)Rln(1-x)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
∆MH° ϭ Ωx(1-x)
The results of the calculations are shown in Figure 6a.
The initial compositions are given on top and the deposited
on bottom of the x-axis. Starting from an initial compo-
sition of Co0.6Fe0.4Si a deposit of Co0.4Fe0.6Si has been cal-
culated at the given conditions. Figure 6b shows the exper-
imental results analysed by EDX and X-ray for compari-
son.
The results show the possibility to calculate the shift of
the composition of chemical transport reactions. The com-
parison of absolute values is not satisfying due to the devi-
ation of the individual values.
4.1.1 Calculations of the transport behaviour in the
system CoSi-FeSi
The shift in the Co/Fe ratio observed by deposition from
the gas phase should be confirmable by model calculations.
The reason for this is a incongruently solution of com-
pounds in the gas phase.
The program TRAGMIN [9] has been used for calcu-
lations. To describe the transport of the silicides for the cal-
culation we have used thermodynamic data of gaseous spec-
ies which are given in [1, 2]. To estimate data for the mixed
crystals CoxFe1-xSi, we used a regular solution with a regu-
larity parameter Ω ϭ Ϫ10 kcal/mol. Calculations with a
ideal solution (regularity parameter Ω ϭ 0 kcal/mol) have
given much worse results. For the calculation the binary
phases FeSi and CoSi are considered as pseudo components
and an arbitrary amount of quasistoichiometric phases
CoxFe1-xSi has been formulated. The program calculates the
equilibrium between the gas phase and one or two “stoi-
chiometric” phases depending on the initial composition.
The deposited composition will be calculated with respect
of the transport conditions, for example the temperature
gradient, compare with [12].
4.1.2 Measurements of the electrical properties in the
system CoSi/FeSi
First resistivity investigations on single crystals are shown
in Figure 7. CoSi shows metallic behaviour whereas FeSi is
comparable with a semiconductor. This results agree well
with literature data [6, 10]. The resistivity of our FeSi crys-
tals is two orders of magnitude higher than the values Kloc
et al. [6] have been observed. The resistivity of the mixed
crystals decreases with increasing cobalt content, see Figure
7a. Due to the irregularities and the small size of the crys-
tals errors at the measurements are likely. Figure 7b shows
the temperature dependence on the resistivity of FeSi. From
the lnρ ϭ f(1/T) plot we got the activation energy of the
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Chemical Vapour Transport of Intermetallic Phases in the System Co/Fe/Si
Fig. 7 (top) Resistivity of CoxFe1-xSi crystals; (bottom) Resistivity
of FeSi crystals
temperature gradient from 1323 K (solution room) to
1123 K and 100 mg iodine as transport agent. On the iron
rich side of the system we observed chemical transport up
to Co0.25Fe0.75Si2. First a transport of silicon has been ob-
served. The second step of the transport is the deposition
of CoxFe1-xSi1 y (MK1), found on the surface of the silicon
crystals [Figure 8b].
Fig.
6 (top) Calculated transport behaviour in the system
CoxFe1-xSi; (bottom) Experimental results of transport in the sy-
stem CoxFe1-xSi analysed by EDX (full line) and X-ray (dashed
line)
In addition needle like single crystals of β-FeSi2 were de-
posited in the middle of the ampoules (Figure 8 c and d)
with dimensions of (4 to 9) x 2 x 0.5 mm3. These β-FeSi2
crystals have been checked by X-ray and EDX, no cobalt
content and no shift of d-spacing have been observed. A
logical conclusion of the coexistence of β-FeSi2 and
conductivity. A value of 19.5 meV was calculated below
160 K. This value is smaller than those given in [6]. Further
investigations are necessary to explain the deviations.
4.2 The system CoSi2/β-FeSi2
CoxFe1-xSi1
CoxFe1-xSi1
(MK1) is that the composition of
(MK1) is the upper phase boundary of
y
Only few transport experiments have been undertaken in
this system. The transport direction from higher tempera-
ture to lower temperature for disilicides is known from own
transport experiments on the binary phases [1, 2] and from
our calculations, too. For the chemical transport reactions
mixtures of iron, cobalt and silicon have been used. The
initial compositions were Co0.05Fe0.95Si2, Co0.25Fe0.75Si2,
Co0.75Fe0.25Si2, Co0.95Fe0.05Si2. With notes from the litera-
ture [6, 7] describing the transport of β-FeSi2, we used a
y
CoxFe1-xSi1ϩy. The residue after 14 days of chemical vapour
transport was CoxFe1-xSi1 (MK1).
y
On the cobalt rich side of the system first deposits were
obtained only after 10 days. The few single crystals ob-
tained, were identified by X-ray as α-FeSi2 (Figure 8 a),
CoxFe1-xSi1 (Figure 8 b ), Si and Fe3Si. The observation
y
of up to four phases simultaneously indicates a non-equilib-
rium state.
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R. Boldt, W. Reichelt, O. Bosholm, H. Oppermann
Fig. 8 Single crystals grown by chemical vapor transport of a) α-
FeSi2, b) CoxFe1-xSi1 y, c) and d) β-FeSi2.
Fig. 10 Chemical vapour transport of β-FeSi2 (1273 K to 1073 K,
agent: 100 mg iodine)
are presented. From previous, tentative experiments we as-
sume that several steady states appear depending on the
composition of source and sink, respectively.
Earlier experiments have shown that FeSi, β-FeSi2, and
α-FeSi2 in the source exist after 4 days of transport. From
such a source mainly silicon and FeSi are deposited at the
beginning of the experiments, see Figure 10 up to point 1.
Later the deposition of FeSi and β-FeSi2 is observed with a
smaller transport rate. At the end of the transport experi-
ment the deposit of FeSi besides and on the surface of sili-
con is situated at the colder end of the ampoule. In the
middle of the ampoule we found a needle-like deposit of β-
FeSi2. The description of the transport behaviour of β-FeSi2
shows some unclearness. Therefore, further investigations
are necessary.
Fig. 9 Chemical vapour transport of FeSi (973 K to 1073 K)
5 Transport Balance
For continuous studies of transport processes a so called
transport balance was built up.
Acknowledgment. The authors wish to thank Dr. M. Dörr (TUD,
Institut für Angewandte Physik und Didaktik der Physik) for elec-
trical measurements, Priv.-Doz. Dr. S. Däbritz and E. Langer
(TUD, Institut für Oberflächen- und Mikrostrukturphysik) for
EDX investigations, Mrs. U. Schmidt (MPI CPfS, Dresden) for
ICP-OES investigations, Prof. Dr. R. Glaum (Universität Bonn) and
Dr. G. Behr (IFW Dresden) for the support of building up the
transport balance. Financial support of the Deutsche
Forschungsgemeinschaft (Projekt Op 62/5-4) is gratefully acknowl-
edged.
Our equipment is comparable with that described in the
literature [11]. The stored data (weight change per time in-
terval) can be smoothed mathematically and represented
graphically. The transport rates are obtained by differen-
tiation of measured data. The functionality of the method
is demonstrated by two examples.
It was possible to show that the chemical vapour trans-
port of FeSi could be described as a steady state transport,
see Figure 9. The transport rates calculated could be con-
firmed.
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Figure 9 shows a representative course of weight change
(a) because of the deposition of FeSi in the sink. Graph b
indicates the incident transport rate as function of time. As
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Figure 10 shows the time course of transport rates of the
chemical vapour transport of β-FeSi2. The transport con-
ditions are the same as given in part 4.2. Graph a shows
again the course of weight change, whereas graph b displays
transport rates. In this graph two stages with different slope
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